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Old 3rd August 2012, 10:38 AM   #221
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Great to see there are stil people reading this topic.

Last week I received my ebay kit and want start building it.

I'm curious, is there a "shortlist" from all the modifications needed to be made with the JHL ebay kit.
I've read almost the whole thread, but the modifications are spread over different messages and are not always the same.

Can anyone help?
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Old 4th August 2012, 12:30 PM   #222
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Hi Scotsky,

Follow milescampbell thread, he made most of modifications and carried out supporting signal testing. I started from a built pcb so i had to strip off/replace components but it should be easier for you to not fit/change components.

I think the most important change is DC offset network on the input transistor. I kept the 10K adjustment pot to allow the DC output offset to be trimmed to within +/- 5mV. I thought the +/- 100mV maximum offset was too high for my 32 ohm headphones so replaced the 3V3 zener with 4 x 1N4148 diodes, 220K replaced by 68K, 100uF replaced with 180uF low ESR and the 1K (4k7 in thread) with 10K. These changes are important to remove any ripple/noise (100Hz) from the input dc bias stage otherwise you will definitely hear mains hum in your headphones. Also replace the 220pF ceramic with Wima or similar MKT. I replaced the 470nF input capacitor with a slightly higher 680nF though 1uF would probably be better. I did not replace the PSU filter capacitors (my board had 2200uF types fitted) because i used a seperate PSU with 2x1500uF/rail low ESR already fitted. The feedback capacitor change from 100uF to 220uF/470uF will probably be less noticeable but still worthwhile if you want to squeeze extra low Hz out of your bass-limited headphones. You must at least turn the original 100uF around otherwise it will be reverse-polarised.

I think its important to remember that the chinese JHL amp was quite a significant departure from the original JHL design based purely on commercial i.e. cost and not high fidelity sonic reasons. The modified headphone amplifier will sound more like the original JHL design.
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Old 7th August 2012, 12:22 PM   #223
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Hi Roclite,

Thanks for explaining in short what are good mod's and what they do.
I can work with that

I've read the whole thread o MilesCampbell, and was sorry to see he stopped continue'ing his findings.
But overall this is a realy nice thread.
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Old 14th August 2012, 07:07 PM   #224
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hi everyone.
i try to build this headphones amp.
which diodes must use (D1 to D4 silicon diodes).
+-12v, circuit 1.
or recommended to build the circuit 2?
thanks in advance.


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Old 15th August 2012, 10:30 AM   #225
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Hi Rob,

The original JLH design used two silicon diodes (1N4148 or equivalent) to give a fixed
DC bias to the input transistor, giving a fixed DC output offset which may be too high for low impedance headphones. The modified or chinese "JHL" design incorporated a zener diode and a 10K potentiometer to allow the output DC offset to be nulled to 0V (typically +/-5mV). This was a bad design choice, i replaced the zener with 4 x 1N4148 silicon diodes and changed other components in the DC bias network to ensure that no mains hum is coupled to the headphones. Refer to the original JLH design to get the correct polarity for the diodes (opposite to the chinese JHL version). You need to set the DC offset over several hours as the output dc offset will be temperature dependant. Circuit 1 can sound very good if you make the proper modifications discussed in this thread. I dont have any experience with circuit 2 and i dont know if it is available as a kit, like circuit 1. Hope that helps.
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Old 15th August 2012, 12:51 PM   #226
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hi roclite!
thanks for answer and help.
no kit, no pcb, i try to build on a protoboard.
usually design the circuit with the help of FidoCad and then bring it back on the protoboard.
the schematic i found here The Class-A Amplifier Site - JLH Headphone Amplifiers is correct, true?
thanks.



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Old 15th August 2012, 12:57 PM   #227
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I forgot: I hope will be able to drive my vintage AKG K340 (400 Ohm).
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Old 15th August 2012, 03:07 PM   #228
oz7aff is offline oz7aff  Denmark
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Default Silk print error

I think that C23 and C24 is shown the wrong way arround on the print on the PCB.
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Old 15th August 2012, 03:22 PM   #229
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That link is correct - JHL original schematic and design explanation. Only the transistors have been substituted for more modern types - the original design is from 1979 - but that should make no noticeable difference to the sound quality. Like most people, i chose the kit because it was very cheap and i could get amp built quickly. The JLH Class A design should be able to drive any headphone with ease. I also use the headphone amp as a pre-amp/buffer between my Sony Bravia TV nicam stereo output and my usual hi-fi setup - JBL Northridge E60/Harrison X-150 Mosfet amp. The sound quality was so good when watching Olympics, you could really feel and hear the whole atmosphere in the venues and it was obvious the normal TV speaker output is heavily filtered/band-limited in comparison. Good luck with your build whatever way you decide to do it and post some pics when completed.
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Old 16th August 2012, 08:58 AM   #230
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@ oz7aff:in the original scheme (circuit 1, jlh 1979) there are not C 23 and c24.
the capacitors ranging from C1 to C8.
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